Here's some progress (using Tadaima Yuusha Boshuuchuu and etripator v0.4).
The following routine writes 1024 bytes at the bank #0 of the memory base 128.
The first 16 bytes are copied from $73b1. Bytes 4 to 15 are the infamous "ﾒﾓﾘﾍﾞｰｽ128" string.
Unfortunately I don't know what the first 4 bytes stands for.

The question is... What are the values of $fc and $fd?
In fact these bytes are set by mb128_read_sectors. It's the sum of the bytes of the sector. So it can be viewed as a CRC.
The header for an empty mb128 is:

I think I figured out the entry structure. Some bytes are still unknown.
The first 1024 bytes of the memory base 128 contains 64 entries of 16 bytes each.
The first entry is the header of the entry list.

byte 0 : CRC (lsb)

byte 1 : CRC (msb)

byte 2 : Total sector count (lsb)

byte 3 : Total sector count (msb)

byte 4:F: Header name (ﾒﾓﾘﾍﾞｰｽ128)

Then follows 63 "directory" entries.

byte 0 : Sector index

byte 1 : Sector count

byte 2 : Unknown (set to 0 and unused by Tadaima Yuusha Boshuuchuu)

byte 3 : Unknown (set to 2 and unused by Tadaima Yuusha Boshuuchuu)

byte 4 : Data CRC (lsb)

byte 5 : Data CRC (msb)

byte 6 : Unknown (set to 0 and unused by Tadaima Yuusha Boshuuchuu)

byte 7 : Unknown (set to 0 and unused by Tadaima Yuusha Boshuuchuu)

byte 8-f : Entry name (set to ﾕｳｼｬM128 in Tadaima Yuusha Boshuuchuu)

More on CRC computation, entry management and all later...
I attached etripator configuration files and output.